Such a known load changeover switch typically has two main current branches and two auxiliary current branches. The first main current branch electrically connects the first winding tap to a load shunt via a vacuum switching tube, the second main load branch electrically connects the second winding tap to the load shunt via a further vacuum switching tube, the first auxiliary current branch connects the first winding tap to the load shunt via a series connection of a further vacuum switching tube and at least one switch-over resistance, and the second auxiliary current branch connects the second winding tap to the load shunt via a series connection of a further vacuum switching tube and at least one further switch-over resistance.
Such a load changeover switch having in total four vacuum switching tubes per phase is known from DE 2021575 A. A respective vacuum switching tube as main contact and a respective further vacuum switching tube that is in series connection with a switch-over resistance, as resistance contact are provided in each of the two load branches respectively connected with a winding tap. In a case of uninterrupted load changeover from the previous winding tap n to the new, preselected winding tap n+1 initially the main contact of the side being switched off is opened and thereupon the resistance contact of the side taking over closes so that a compensating current limited by the switchover resistors flows between the two winding taps n and n+1. After the previously closed resistance contact of the side switching off has opened, the main contact of the side taking over then closes so that the entire load current leads from the new winding tap n+1 to the load shunt; the changeover is thus concluded.
However, in different cases of use of such known on-load tap changers with vacuum switching tubes for regulation of power transformers a high surge-voltage strength of up to 100 kV and significantly above that is required. Such undesired surge voltages, the level of which is substantially dependent on the construction of the tapped transformer and the winding parts between the individual tap steps, are on the one hand lightning surge voltages that result from lightning strikes in the mains. On the other hand, switching surge voltages can also occur that are caused by unpredictable switching surges in the mains to be regulated. In the case of insufficient surge-voltage strength of the on-load tap changer a transient step short-circuit or undesired disruption of the ceramic or the damping screen of vacuum switching tubes in the load branch not conducting the load current can occur, which not only can cause long-term damage thereof, but is generally undesirable.
In order to combat excessive surge-voltage loads it is already known from DE 2357209 [U.S. Pat. No. 3,934,174] and DE 2604344 to provide protective spark gaps or voltage-dependent resistors or both between the load branches; however, these means are, in various cases, insufficient and are unable to exclude or completely exclude harmful surge voltage loads in their effect.